1. Field of the Invention
The present invention pertains generally to evaluation of optical characteristics of transparent glazing products, and more particularly to evaluation of automotive glass for compliance with established standards of optical quality. The invention has particular utility in the evaluation of windshields for incorporation in so-called automotive head-up display systems.
2. Description of the Prior Art
Head-up display systems are becoming increasingly popular as a safety and convenience feature on present day automotive vehicles. In such systems, vehicle operating information is projected onto a vehicle windshield and images are reflected into the operators forward field of vision. The operator can thus observe the field in front of the vehicle and the displayed operating information simultaneously. Head-up display systems have heretofore been utilized extensively in the aircraft industry, and to a lesser extent in the automotive field. Use of the system in the automobile minimizes the need for the operator to divert his or her attention from observation of the road ahead to make periodic observations of the dashboard display panel. The operator is therefore able to continuously focus attention on the road while simultaneously viewing essential vehicle operating information such as speed, fuel level, engine temperature, etc., thereby greatly enhancing the safety of vehicle occupants and others.
Despite the advantages of head-up display systems, in order for their use in automobiles to become widespread it will be necessary for the windshields employed in the systems to meet the stringent cost and quality requirements of the automotive industry. Present day automotive windshields comprise two sheets of curved glass integrally bonded to an interlayer of plastic, generally a sheet of polyvinyl butyryl. The geometry of the windshield unit is of critical importance in establishing the optical quality of the head-up display system. The windshield is, of course, installed in the vehicle in an inclined position so that the driver views the road ahead at a significant angle to the surfaces of the windshield.
The vehicle operating information is projected onto the windshield by a projector mounted beneath the dashboard of the vehicle, and is reflected from the inboard surface as a primary display image for viewing by the driver. A secondary display image of the projected information, of lesser intensity, is reflected from the outboard surface of the windshield. Because of the geometry of the windshield construction and the angle of installation in the vehicle, the secondary image may be displaced from the primary image as viewed by the driver so as to create a ghosting effect and make reading of the information difficult. In order to alleviate this problem, it has been proposed to fabricate the windshield so that the inboard and outboard surfaces, at least in the display area, are not parallel. In other words, the surfaces are disposed at a slight angle to one another, the angle being calculated in accordance with the angle of installation of the windshield to cause the primary and secondary reflected images as viewed by the driver to be substantially superimposed upon one another. This can be accomplished as disclosed, for example, in U.S. Pat. No. 5,013,134, by laminating the glass sheets to a sheet of interlayer material which is of a suitably tapered or wedged configuration.
Evaluation of head-up display windshields for compliance with established standards involves grading the unit in accordance with a number of performance criteria. Among the criteria are the relative head-up display image size and the amount of displacement between the images reflected from the inboard and outboard surfaces of the windshield. These criteria are functions of a number of parameters, including the vertical radius of curvature of the curved windshield in the display area, the wedge or non-parallelism between the inboard and outboard surfaces, and the windshield's slope, or angle of installation in the vehicle. Heretofore, evaluation has generally been accomplished by using an actual projector to simulate the environment in which the windshield is to be later employed in a vehicle, and then observing and evaluating the image produced. The head-up system, comprising both the projector and the windshield, is thus evaluated. When installed in a vehicle with a different projector, the display may differ considerably from that in the test system. A suitable system has not been available heretofore for quickly and accurately ascertaining the values of the pertinent parameters for the windshield per se, and evaluating the collected data to determine whether the head-up display area of the windshield complies with the established standards.